To assess the incidence, nature and evolution of cardiac disease in Duchenne muscular dystrophy, 328 patients were studied between 1976 and 1987 for periods varying from 3 to 11 years. Patients underwent regular clinical examination, electrocardiography, echocardiography and radiological assessment. Pre-clinical cardiac involvement was found in 25% of patients under 6 years old increasing to 59% between the ages of 6 and 10 years and then declining in incidence with age. Clinically apparent cardiomyopathy is first evident after 10 years of age and increases in incidence with age, being present in all patients over 18 years of age. Its clinical impact is discussed.